Difficulty showing that a group G applied on X, G_{x}

Jason Yuhas

Jason Yuhas

Answered question

2022-01-15

Difficulty showing that a group G applied on X,Gx(xX) and Gy,wyG(x) are same iff Gx is a normal subgroup of G

Answer & Explanation

Joseph Lewis

Joseph Lewis

Beginner2022-01-16Added 43 answers

Let GX be an action of a group on a set. For a point xX the stabilizer of x is defined as Gx:={gG:gx=x}. It is trivial that Gx are subgroups of G: indeed, if g, hGx then since hx=x we have x=h1x so gh1x=gx=x, and thus gh1Gx.
Now let xX be a point and assume that for any yorb(x) we have Gx=Gy. This of course means that Gx=Ggx for all gG. We show that Gx is normal;
Let gGx and hG. We have to show that hgh1Gx. But, note that Gh1x=Gx, so gGh1x. We compute: hgh1x=h(g(h1x))=h(h1x)=x, so hgh1Gx, proving normality.
Conversely, assume that Gx is normal and let yorb(x),say y=gx for some fixed gG. We need to show that Gx=Ggx. Let hGx. Then g1hgGx by normality, so g1hgx=x,so hgx=gx,so hGgx, showing that GxGgx. Now let hGgx; then hgx=gx,so g1hgx=x,so g1hgGx. But since Gx is normal, we also have g(g1hg)g1Gx,i.e. hGx.

Melinda McCombs

Melinda McCombs

Beginner2022-01-17Added 38 answers

Step 1
Suppose Gx is a normal subgroup. Then g1Gxg=Gx, from which we see that if gyGy  g1gygx=x. But g1gyg is thus in Gx and thus also gg1gygg1=gyGx.
Also gxGx if and only if for each g we get gxgx=gx (again because g1gxgx=x), so gxGy.
Step 2
Now suppose Gx=Gy. Then we get gygx=gx and gyx=x. Thus we get gygx=ggyx and thus g1gygx=x. Thus for each gyGy=Gx we get g1gygGx=Gy.
But I think this is as far as this goes now. We can get normality if we demand that Gx=Gy for all yG(x).
EDIT: In fact we need this. Suppose g=e,then y=x. As Gx=Gx it is easy to see that the given premisses are not suffiecient to get normality.

Do you have a similar question?

Recalculate according to your conditions!

Ask your question.
Get an expert answer.

Let our experts help you. Answer in as fast as 15 minutes.

Didn't find what you were looking for?